1022 

T2.I5 


THE  CORROSION  OF  METALS  IN 
ORGANIC  ACIDS 


BY 

JOHN  BRADSHAW  TAYLOR 


THESIS 


FOR  THE 

DEGREE  OF  BACHELOR  OF  SCIENCE 

IN 

CHEMICAL  ENGINEERING 


COLLEGE  OF  LIBERAL  ARTS  AND  SCIENCES 


UNIVERSITY  OF  ILLINOIS 


1922 


T2  15 


UNIVERSITY  OF  ILLINOIS 


M&yjo i92_?_ 


THIS  IS  TO  CERTIFY  THAT  THE  THESIS  PREPARED  UNDER  MY  SUPERVISION  BY 


•John  Bradshaw  Taylor 


The  Corrosion  of  Metals  in  Organic  Acids. 
ENTITLED 


IS  APPROVED  BY  ME  AS  FULFILLING  THIS  PART  OF  THE  REQUIREMENTS  FOR  THE 


DEGREE  OF 


Bachelor  of  Science. 


In 

Ihemical  Engineering 


College  of  Liberal  Arts  and 


Sciences. 


Approved  : 


HEAD  OF  DEPARTMENT  OF 


500196 


.1 


" •- 


•n  -• 


Acknowledgment. 

The  author  wishes  to  express  his 
sincere  thanks  to  Dr. -J.  E.  Reedy, 
who  has  proposed  this  problem 
and  helped  in  the  work  with  his 
ready  suggestions. 


Index  of  Contents. 

Page 

I.  Introduction — 1 

II.  Historical 2 

III.  Experimental 3 

IV.  Discussion — 10 

V.  Summary — — 13 

VI.  Bibliography — * 14 


Digitized  by  the  Internet  Archive 
in  2015 


https://archive.org/details/corrosionofmetalOOtayl 


THE  CORROSION  OF  METALS  IN  ORGANIC  ACIDS. 

I 

Introduction. 

/ The  corrosion  of  metals  in  organic  acids  is  of  importance 
in  deciding  the  material  most  suitable  for  containers  and  apparatus 
for  the  handling  of  fruit  j uices, canned  fruits, and  milk  etc.  In  an 
extensive  collection  of  lata  from  many  analyses^  it  has  been  shown 
that  tartaric, malic, succinic, and  citric  acids  are  found  in  fruits 
to  the  exclusion  of  almost  all  others.  The  acids  investigated  in 
this  work  are  confined  to  the  acids  found  in  fruits.  The  toxicity 
of  the  metal  salts  and  the  resistance  of  the  metals  to  corrosion 
must  be  taken  into  consideration.  In  general  copper  is  more  toxic 
than  tin  and  tin  more  toxic  than  zinc, though  the  toxicity  may 
change  with  the  particular  salt  formed. 

The  amount  of  corrosion  is  commonly  said  to  vary  or  be  inf- 
luenced b*y  several  conditions: 

1. -  The  facts  of  corrosion  are  often  explained  by  employing  the 
electrolytic  potentials  of  the  metals.  A more  electro  negative 
metal, (using  the  convention  adopted  by  the  American  Electrochemical 
3ociety)is  supposed  to  corrode  more  than  one  that  is  less  electro- 
negative. Thus  the  electronegativity  or  electrolytic  solution 
pressure  is  a measure  of  the  tendency  to  form  positive  ions, leaving 
the  metals  negatively  charged. 

2. -  The  physical  treatment  of  the  metal  influences  corrosion. 

Any  bending  or  twisting  will  usually  cause  increased  corrosion  at 


x' 


■ 


the  point  so  strained. 

3. -  The  effect  of  impurities  in  metals  may  increase  or  decrease 
the  corrosion, depending ( as  it  is  usually  explained)on  whether  their 
effect  is  to  make  the  metal  itself  more  or  less  electro-negative. 

4. -  The  effect  of  corrosion  products  is  recognized  to  be  an  imp- 
ortant  item  in  corrosion, b ut  the  specific  cause  is  rather  vague. 

The  setting  up  of  a "galvanic  cell"is  used  by  Richardson^  in  explain! 
ing  the  effect  of  iron  rust  in  increasing  corrosion.  Bailey^  in  | 

his  work  with  aluminium  has  noticed  the  opposit  or  protective  eff- 
ect of  the  oxide  coating  formed. 

5. -  Dissolved  atmospheric  oxygen  may  either  attack  the  metal 
directly  or  assist  in  attack  by  removal  of  hydrogen.  That  is, a metal 
may  corrode  in  either  or  both  of  two  ways. 

Me  + 2H2=toe°°+  H2  and  H2  + 0=  H20 
or  Me  + 0 = MeO  , directly. 

6. -  The  structure  of  the  acid  may  also  affect  the  amount  of 
corrosion. 

The  present  work  has  been  carried  on  in  order  to  find  the  metal 
most  resistant  to  corrosion  of  those  tested, and  to  investigate  the 
possible  effects  of  the  amino  group  and  of  the  number  of  hydroxyl 
groups  on  the  amount  of  corrosion. 

11  ! 
Historical. 

The  corrosion  of  metals  in  dilute  organic  acids  has  been  in- 
vestigated(1921)by  Jean  G. Shepherd.  An  attempt  to  measure  the  com- 

0 

parative  rate  of  corrosion  was  made  by  immersing  the  metal  in  the 


1 


’ 


*■ 


-3- 

forra  of  strips  in  a soultion  contained  in  cells  with  mercury  man- 
ometer attachments.  This  afforded  a means  of  detecting  any  absorb- 
tion  of  oxygen  or  evolution  of  hydrogen, and  thus  a measure  of  the 
rate  of  corrosion, by  noting  the  difference  in  level  of  the  mercury. 
Readings  were  made  every  day  for  ten  days.  The  atmosphere  above  the 
liquid  was  sparked  to  determine  any  hydrogen  evolved.  In  this  way 
it  was  hoped  to  ascertain  the  true  amount  of  oxygen  absorbed. 

Zinc, t in, copper , and  aluminium  were  tested  in  propionic, tartaric, 
citric, and  in  alanine.  This  method  apparently  should  have  been  very 
reliable, but  the  results  obtained, though  admitting  of  some  comoar- 
isons, were  of  almost  no  value.  The  chief  reason  for  this  probably 
lies  in  the  fact  that  the  sparking  of  the  hydrogen  when  it  is  pres- 
ent in  very  small  amounts  is  uncertain.  Further  explanation  is  found 
in  certain  conclusions  drawn  as  a result  of  this  work. 

Ill 

Experimental. 

(a ) Method- 

The  method  used  here  consisted  in  weighing  the  strip  of  met- 
al before  immersing  in  the  reagent.  On  removal, after  careful  clean- 
ing, drying,  and  reweighing, the  loss  in  weight  was  a measure  of  the 
amount  of  corrosion.  Zinc, tin, copper, and  aluminium  strips  were  used 
all  having  the  same  surface  area=18.9  sq.cm.  They  were  all  approx- 
imately 5*5  cm. long, 1.7  cm. wide, and  9*15  cm. thick.  After  cutting 
to  this  size  the  strips  were  polished  with  fine  emery  to  make  the 
surfaces  as  uniform  as  possible.  These  were  immersed  in  a cell  or 
bottle  with  50  c.c.of  the  acid  solution(see  fig.l)and  placed  on  a 


. 


Co  rros/on 


JBott/e 


-4- 

shaking  platform  to  gently  agitate  the  liquid.  At  the  end  of  a per- 
iod of  seven  days  the  strips  were  taken  out , dried, and  afterwards 
cleaned  with  a soft  bristle  brush  to  remove  any  products  of  corr- 
osion. 

A few  difficulties  were  encountered  in  making  the  tests. 

The  acids  used  would  form  mould  on  standing  for  a few  days  and  thy- 
mol was  successfully  used  to  prevent  this.  However  when  copper  was 
tested  in  tartaric  and  in  malic  acids  a black  tarry  deposit  was 
formed  on  the  metal  and  on  the  sides  of  the  cell.  This  gave  no  test 
for  copper, was  soluble  in  ether  or  benzene, and  was  presumably  a 

formation  caused  from  or  by  the  thymol.  This  was  shown  to  be  the 
case  by  trying  fresh  acid  without  thymol.  No  tarry  deposit  was 
given  and  the  corrosion  loss  was  less.  No  reason  could  be  found 
for  the  formation  with  copper  nor  for  its  absence  with  the  other 
metals.  Formaline  (four  drops  to  500  c.c.sol.)  was  substituted  for 
thymol  and  no  further  trouble  was  had.  Both  thymol  and  formaline 
were  tested  for  their  corrosive  effect  on  the  metals  by  immersing 
in  pure  water  as  well  as  in  pure  water  plus  thymol  or  formaline.  It 
was  found  to  be  nil. 

( 2 ) Results- 

TARTARIC  ACID. 

Zinc  in  tartaric  acid  formed  a white  product  which  adhered 
to  the  strips  and  which  was  easily  brushed  off  after  drying.  The 
amount  of  corrosion  varied  with  the  concentration  of  the  acid  ex- 
cept in  N/100  and  N/1000  acid, the  zinc  being  corroded  to  a greater 
extent  in  N/1000  than  in  N/100  acid.  This  was  noted  by  Shepherd. 


. 


4 ■ ! S J KM II  ' 'Jj  ^ 


Table  of  the  Data. 


Tin 

TARTARIC 

Zinc 

ACID 

Topper 

Aluminium 

N/l 

.1677 

.1965 

.0891 

.0024 

.1535 

.1906 

.0911 

.0025 

N/10 

.5372 

.0320 

.0825 

.0020 

• 5363 

. 0854 

.0324 

.0019 

N/100 

.0350 

.0275 

.0318 

.0021 

.0821 

.0273 

o 

O 

• 

. 0022 

N/lOOO 

o 

r- 

o 

o 

.0593 

.0039 

.0007 

.0072 

.0645 

.0037 

.0007 

Tin 

MALIC  AC 
Zinc 

ID 

Copper 

Aluminium 

N/l 

.5865 

1.7584 

.2036 

.0013 

• 5875 

1. 7520 

.1907 

.0020 

N/10 

. 3660 

. 1668 

.1144 

.0015 

.3591 

.1672 

.1155 

.0020 

N/100 

.0132 

.0165 

.0237 

.0022 

.0132 

. 0865 

.0241 

.0015 

N/1000 

.0010 

. 0031 

.0028 

0 

0 

0 

-a 

.0008 

.0033 

.0023 

.0009 

SUCCINIC 

ACID 

Tin 

Zinc 

Copper 

Aluminium 

N/l  .0843 

. 4429 

.1902 

.0011 

.0823 

.4279 

.1955 

.0019 

N/10  .0127 

.1306 

.0971 

.0015 

.0143 

.1230 

.0375 

.0017 

N/100  .0019 

.0390 

.0107 

.0022 

.0020 

.0410 

.0103 

.0011 

N/1000  .0007 

.0062 

.0003 

.0003 

.0004 

.0063 

.0003 

.0004 

ASPARTIC 
Tin  Zinc 

ACID 

Copper 

Aluminium 

N/100  .0046  .0205 

.0149 

.0146 

.0040  .0193 

.0151 

.0157 

N/1000  .0011  .0042 

.0014 

.0006 

.0012  .0037 

.0013 

.0007 

-5- 

To  explain  this  apparent  irregularity  the  amount  of  zinc  tartrate 
corresponding  to  the  amount  of  acid  in  the  solution  was  calculated 
and  found  to  be  less  than  the  actual  corrosion  loss.  Assuming  the 
'acid  to  have  been  entirely  used  up, the  additional  corrosion  may  be 
due  to  the  action  of  water  on  the  zinc, which  is  considerable.  A 
strip  of  zinc  tested  in  water  alone  gave  a pitted  appearance  after 
a slight  corrosion  coat  had  been  removed.  This  coating  ani  pitting 
were  also  present  in  the  case  of  zinc  and  N/1000  acid  and  account 
for  the  greater  corrosion  found.  The  coat  was  also  much  more  adher- 
ent than  in  the  other  concentrations  ani  seems  to  have  had  a catal- 
ytic effect  on  th  corrosion. 

In  some  cases  however  the  coating  may  be  protective.  This  is 
particularly  true  in  the  case  of  Aluminium. 

Aluminium  in  tartaric  acid  was  corroded  only  negligibly. 
Bailey5  has  observed  this  in  his  work  on  aluminium.  Aluminium  has 
the  property  of  forming  a coating  of  A1(0H)3  on  its  surface  when 
in  contact  with  a liquid.  This  coating  acts  not  as  a catalystybut 
interferes  with  the  direct  action  of  J he  r agent  on  metal. 

This  oxidation, probably  by  dissolved  oxygen, is  a type  of  passivity, 
and  has  been  observed  by  Ric hards on z in  connection  with  the  corr- 
osion of  iron.  Aluminium  was  also  only  corroded  negligibly  in 
malic, succinic, and  aspartic  acids.  The  results  are  almost  identical 
with  those  for  tartaric  acid  ani  show  that  the  oxide  coating  formed 
is  the  only  factor  in  the  corrosion  of  aluminium. 

Tin  in  tartaric  acid  formed  an  insoluble  product  in  N/10 
acid  where  a white  coating  remained  on  the  strips.  The  corrosion 


. 


. • 


. 


-6- 


as  a result,  of  this  coating  was  greater  than  in  N acid  where  the 
product  was  soluble  and  had  no  opportunity  to  settle  out  on  the  sur- 
face of  the  metal.  The  product  in  N/100  and  N/1000  was  slight  and 
the  corrosion  loss  correspondingly  less.  Tin  in  N/100  and  N/1000 
acid  gave  more  loss  than  could  be  accounted  for  by  the  amount  of 
acid  present.  ( i.  e . as  tin  tartrate)  This  is  evidience  of  the  form- 
ation of  some  sort  of  a complex  tin  tartrate, since  tin  is  not  corr- 
oded in  water  ai  all.  A slime  or  scale  on  the  strips  indicated  the 
presence  of  oxides  of  tin, which  may  also  have  increased  the  amount 
of  corrosion. 

Copper  in  N/l  and  N/10  tartaric  acid  gave  a clear  blue  sol- 
ution with  a blue  product.  The  strip  was  clean  in  N acid  but  the 
precipitate  adhered  in  N/10  acid.  This  may  account  for  the  slight 
difference  in  corrosion  loss  between  N and  N/10  acids.  In  N/100 
and  N/1000  a blue  solution  resulted, but  no  product  was  precipitated 
and  the  strips  were  darkened.  This  oxide  coating  was  ascribed  to 
direct  oxidation  by  the  dissolved  oxygen, which  in  the  more  concen- 
trated solutions  was  removed  by  union  with  the  H°. 

The  order  of  corrosion  in  tartaric  acid  was  as  follows  in 
Table  I, the  order  being  that  which  would  be  predicted  from  the 
electrolytic  potentials  of  the  metals.  Zinc, most  corroded, is  more 
electro-negative  than  tin, and  tin  more  so  than  copper.  Aluminium  is 
more  electro-negative  than  zinc, but  this  factor  is  overcome  by  the 
protective  coating.  The  exception  to  this  order  in  N/10  acid  has  been 
expl ainea  by  the  coating  formed' on  tin  in  N/10  acid  which  raises  its 
corrosion  loss  to  an  abnormal  amount.  In  N/100  acid, tin  and  copper 


-7- 

Preoeed  zinc, possibly  due  to  the  formation  of  the  oxide  coatings 
observed  on  both  tin  and  copper. 

Table  I. 

Order  of  Corrosion  in  Tartaric  Acid. 

OonCj. Order^ 

N/l Zn-Sn-Ou-Al 

N/10 Sn  ( Zn-Cu- ) A1 

N/100 Sr ( Zn-Cu- )A1 

N/1000 Zn-Sn-Ou-Al 

( — — ) indicates  practically  equal  corrosion  loss. 

MALIC  AOID. 

Zinc  in  N/l  malic  acid  formed  a voluminous, white, spongy  pre- 
cipitate. Part  of  this  adhering  to  the  strips, took  particles  of 
the  metal  with  it  when  "peeled"  from  the  strips  which  were  deeply 
pitted.  This  product, probably  zinc  malate  hydrolyzed  by  water,  was 
soluble  when  more  normal  malic  acid  was  added.  In  the  other  conc- 
entrations the  product  was  compact  and  powd.ery,and  easily  brushed 
from  the  strips.  No  pitting  was  evidenced. 

Tin  in  malic  acid  formed  a clear  yellow  solution  with  no 
precipitate  in  normal  acid.  In  the  other  concentrations  a white  prod 
uct  was  formed  soluble  in  normal  acid.  No  coatings  adhered.  The 
corrosion  varied  with  the  concentration. 

Cooper  in  N and  N/10  malic  acid  formed  blue  solutions  with 
a blue  product  precipitated  which  could  be  removed  from  the  strips 
when  dry, while  in  N/lOOOand  N/1000  the  strips  were  darkemed  slight- 
ly and  no  product  was  precipitated.  The 


corrosion  varied  with  the 


. 


-8- 


conoentration. 

The  order  of 
Table  II.  The  order 
but  shifts  occur  in 
crease  in  corrosion 


corrosion  in  malic  acid 
in  N acid  follows  the  el 
the  other  concentrations 
loss  as  compared  to  zinc 


is  as  follows  in 
ectrolytic  potentials 
. Copper  tends  to  in- 
and  tin. 


9 


Table  II. 

The  Order  of  Corrosion  in  Malic  Acid. 

£oncA .Crder^ 

N/l Zn-Sn~Cu-Al 

N/10 Sn-Zn-Cu-Al 

N/100 Cu-Zn-Sn-Al 

N/1000------ — — ( Zn-Cu ) ( Sn-Al ) 


SUCCINIC  ACID. 

Zinc  in  succinic  acid  gave  an  insoluble  white  product  which 
settled  on  the  strips.  The  amount  of  corrosion  varied  with  the  con- 
centration from  N/l  to  N/1000  acid. 

Tin  in  succinic  acid  formed  an  insoluble  white  product  and 
coating.  The  ammount  of  corrosion  varied  with  the  concentration  and 
in  N/1000  acid  was  negligible. 

Copper  in  succinic  acid  formed  a vivid  blue  product  and  sol- 
ution in  N/l  and  N/10  acid.  These  strips  were  heavily  coated  with 
the  product.  With  N/100  and  N/1000  acid  the  solution  was  light  blue 
and  the  strips  were  darkened.  No  product  was  precipitated.  This 
fact  accounts  for  the  dark  oxide  coating  caused  by  dissolved  oxy- 
gen which  was  prevented  in  the  more  concentrated  solutions--.! ust  as 
happened  with  copper  in  N/100  and  N/1000  malic  and  tartaric  acids. 


' 


III. 


-9- 

The  order  of  corrosion  in  succinic  acid  is  as  in  Table 

Zinc  was  corroded  most  in  every  concentration.  The  heavier  coating 
observed  on  copper  indicating  a lesser  solubility  of  the  copper 
corrosion  product  than  that  of  the  tin  may  account  for  the  position 
of  copper  ahead  of  tin, although  tin  has  the  greater  electrolytic 
solution  pressure. 

Table  III. 

The  Order  of  Corrosion  in  Succinic  Acid. 

_Conci Ordgr^ 

N/i — Zn-Cu-Sn-Al 

N/10 Zn-Cu-Sn-Al 

N/100 — — Zn-Cu-Sn-Al 

_JlLlOOO~zzzzzzzzzZniQuz£ulkl 

ASPARTIC  ACID. 

(The  solubility  of  aspartic  acid  did  not  allow  the  preparat- 
ion of  N/l  and  N/10  solutions. ) 

Zinc  in  aspartic  acid  gave  a slight  white  product  on  the 
strips  which  was  removed  on  drying. 

Tin  gave  clear  solutions  with  clean  strips.  The  corrosion  was 
small  in  each  case. 

Copper  formed  clear  solutions  with  an  insoluble  blue  product. 
The  strips  were  slightly  darkened. 

Aluminium  in  N/1000  aspartic  acid  was  negligibly  corroded, 
but  in  N/100  the  corrosion  was  appreciable.  This  departure  could 
not  be  accounted  for. 


' 


-10- 

The  order  of  corrosion  in  aspartic  acid  is  as  the  electro- 
lytic potentials  would  predict. in  N/1000  solution, but  in  N/100  the 
copper  may  be  supposed  to  form  a complex  ion  with  the  amino  group 
of  the  acid, somewhat  as  it  forms  the  ion  Cu(NH3)4—  — , since  copper 
preceeds  tin  in  this  concentration. 

i 

fable  IV. 

The  Order  of  Corrosion  in  Aspartic  Acid. 

ConcA Orders 

N/199 — ■ Zn-Cu-Al-Sn 

N/1000--------  zZn-£!n-Gu-Al_ 

IV. 

Discussion. 

When  the  results  with  the  four  acids  are  examined  the  tend- 

* 4 

encies  in  each  case  are  seen  to  be  practically  the  same.  (See  tables 
I-IV)  That  is  in  more  concentrated  solutions  , N/l  and  N/10,  the 
order  of  corrosion  follows  closely  the  respective  electrolytic 
potent ials, except  in  the  few  cases  where  the  effect  of  coatings  has 
been  observed.  In  N/100  and  N/1000  solutions, however, shifts  occur, 
the  most  evident  of  which  is  the  increased  comparative  corrosion  of 
copper.  A copper  oxide  coating  was  formed  in  every  N/100  and  N/1000 
acid  solution.  Tin  in  the  same  concentrations  was  often  coated  with 
a slime  or  scale  of  oxide.  The  effect  of  coatings  in  both  concentra- 
ted and  dilute  solutions  seems  to  be  catalytic (except  in  the  case 
of  aluminium ), and  may  be  best  explained  by  saying  that  the  coating 
in  either  a finely  divided  or  colloidal  condition, lying  against 
the  surface  of  the  metal  acts  as  a sponge  to  t heccorrosive  reagent. 


. 

f 


■ 


-11- 

greater  concentration  than  usual  at  the  surface  of  the  metal 

and  increasing  corrosion.  Copper  in  this  way, on  account  of  its  oxide 
coating  , is  placed  ahead  of  tin  in  dilate  solutions  and  sometimes 
is  even  ahead  of  zinc.  The  heavy  coating  formed  with  zinc  in  normal 
malic  acid  is  a striking  example  of  the  effect  of  xuch  "spongy"  coat 
ings  in  the  more  concentrated  solutions. 

The  comparative  effect  of  the  acids  on  the  four  metals  is 
interest ing.  ( see  Table  V).0ne;of  the  factors  that  influence  this  ordei 
has  already  been  mentioned, namely-the  structure  of  the  acid.  Tar- 
taric acid  with  its  two  hydroxyl  groups  might  be  expected  to  be 
more  corrosive  than  malic  with  one  hydroxyl  and  suocinic  with  none. 
Also  the  amino  group  of  the  aspartic  acid  would  have  its  effect  in 
some  degree.  The  other  factor  is  the  degree  of  ionization  of  the 
different  acids.  These  are  given  in  Table  VI  and  were  calculated 
from  the  dissociation  constants5  using  Ostwald's  Law  for  ionization 
As  a monobasic  acid. 

The  order  of  effect  on  tin  and  copper  in  N/100  and  N/1000 
acids  is  tartaric, malic, aspartic, succinic.  This  is  in  accordance 
with  the  degrees  of  ionization  and  with  the  number  of  OH  groups. 
Tartaric  acid  is  most  highly  ionized  and  posseses  the  greatest  num- 
ber of  OH  groups.  However  in  N/l  and  N/lOand  with  zinc  in  every  con- 
centration, the  order  of  corrosive  effect  is  changed.  This  is  probab- 
ly on  account  of  the  greater  effect  of  the  corrosion  products  formed 
in  the  more  concentrated  solutions.  In  these  solutions  where  appre- 
ciable coating  has  been  formed  the  effect  is  not  the  same  with  the 


' 


-12- 

different  metals.  For  example, succinic  acid  which  has  the  least 
corrosive  action  on  tin  in  the  more  concentrated  solutions, is  sec- 
ond in  its  effect  on  copper  and  zinc  in  the  same  concentrations. 
This  is  because  of  the  greater  comparative  effect  of  the  coatings 
on  copper  and  zinc  in  increasing  the  amount  of  corrosion. 

The  effect  of  the  amino  group  is  to  place  aspartic  acid 
between  succinic  acid  with  no  OH  groups  and  malic  with  one. 

The  formation  of  complex  metallic  ions  such  as  the  basic 
copper  tartrate  by  hydrolysis  in  solutions  of  low  H°ion  concen- 
tration may  be  a factor  in  the  irregularities  noted, but  there  is 
nothing  positive  to  indicate  their  formation. 

Table  V. 


Comparative 

effect  of 

the  Acids 

on  the  Metals. 

Zinc 

Tin 

Copper 

Aluminium 

N/l 

M-S-T— 

M-T-3-- 

M-S-T- — 

(— - 

N/10 

M-S-T — 

T-M-3-- 

M-S-T — 

N/100 

S-T-A-M 

T-M-A-S 

T-M-A-3 

----- 

N/1000 

T-S-A-M 

T-M-A-S. 

T-M-A-S 

Table  VI. 

Degrees  of  Ionization  {%) 
N/l  N/10  N/100  N/1000 

Tartaric 

3.1 

9.4 

26.  3 

50.0 

Malic 

2.0 

6. 1 

18.0 

45.0 

Aspartic 

— 

— 

10.8 

30.  2 

Succinic 

CO 

• 

o 

f 

2.  6 

* 

~<r 

00 

22.5 

. 


-13- 

IV 

Summary. 

1. -  Aluminium  is  most  resistant  to  corrosion  and  is  only  neglig- 
ibly corroded  in  such  organic  acids  as  tart aric, malic, succinic, and 
aspart ic; probably  on  account  of  the  formation  of  a protective  oxide 
coating. 

2. -  Zinc  is  least  corroded  in  these  acids, followed  by  tin  and 
copper, though  this  order  is  subject  to  wide  changes  in  certain  cases 

3. -  Electrolytic  solution  pressure  governs  the  comparative 
amount  of  corrosion  of  the  metals  until  disturbing  factors  enter, 
the  most  important  of  which  is: 

4. -  The  effect  of  a coating  of  the  product  formed  on  the  metal, 
which  is  catalytic  to  corrosion  except  in  the  case  of  aluminium, 

by  acting  as  a sponge  to  the  corrosive  reagent  close  to  the  surface 
of  the  metal. 

5. -  The  amount  of  corrosion  increases  with  the  number  of  hydroxyl 
groups  in  dilute  solutions  of  the  acids, but  in  more  concentrated 
solutions  the  effect  of  coatings  disturbs  this. 

6.  -Corrosion  increases  with  the  degree  of  ionization  of  the 
different  acids  in  dilute  solutions, but  here  also  in  more  concent- 
rated solutions  this  factor  is  overcome  by  the  effect  of  the  corro- 
sion product. 

7. -  The  amino  group  increases  the  corrosive  effect  of  aspartic 
acid  over  that  of  succinic  acid, which  has  neither  OH  nor  NH2  group. 

8. -  The  evidence  of  the  formation  of  complexes  is  negative, al- 
though their  formation  seems  possible. 


' 


-14- 

9.-  The  changing  concentration  of  the  acid  used, its  entire  con- 
sumption in  some  instances, the  varying  types  and  effects  of  corros- 
ion products, and  the  possibility  of  other  disturbing  conditions, 
make  the  successful  application  of  the  manometer  cell  method  for  the 
determination  of  corrosion  rate  practically  impossible. 

V 

Bibliography. 

1.  W.D. Bigelow  and  P. B. Dunbar,  J.I.E.C.  IX-762-1917- 

2.  W. D. Richardson,  Chera. and  Met . 23-23-1920. 

3.  3. 1. Bailey,  -J.  S.  C.  I.  32-118T-1920. 

4.  Thesis-University  of  Illinois-1921. 

5.  Uandolt  and  Born Phys. Them. Tabellen. 


